1. Field of the Invention
The present invention relates to a novel vinyl-substituted benzylorganotin compound, a polymer using it, a process for preparing the polymer and a novel rubber composition.
2. Description of the Related Art
Generally, industrially produced polymers are diversified into various kinds because of diversified requirements in each field of application. A polymer suited to a particular application is selected from among the various kinds and used according to requirements of the particular situation. For the purpose of providing polymers suited to various applications, various constituting units in polymers have been developed. Vinylaromatic hydrocarbons, such as styrene, are used as materials for coating compounds, drying oils, polymeric materials (rubbers, resins and the like) and the like. In particular, they are widely used industrially as material monomers of useful polymeric materials, such as polystyrene resins, rubber compounded high-impact polystyrene resins, acrylonitrile-butadiene-styrene copolymer (ABS), styrene-acrylonitrile resin (SAN), styrene-butadiene copolymer (SBR) and the like, with use of catalysts, such as radical polymerization catalysts, cationic polymerization catalysts, anionic polymerization catalysts and coordination catalysts.
On the other hand, tin compounds are known to provide various kinds of useful functional compounds. Among the tin compounds, inorganic tin compounds have been used as surface treatment agents for non-conductive materials such as glass, additives for reducing light transmission of ceramics and glass, electric plating material, tooth paste and the like. Organotin compounds have been widely used in industry as insecticides, agricultural chemicals, wood preserving agents, coating compounds for ship bottoms, anticorrosion agents for marine products, antistaining coating materials, fungicides, stabilizers for polyvinyl chloride, antioxidants for rubber, lubricants, various kinds of catalyst and the like. Organic functional groups bonded to tin, particularly chemical compounds containing benzyl-tin linkage, have been shown to be effective for modification of polymeric materials such as rubber because of the radical reactivity thereof and are industrially attracting attention.
Vinyl-substituted benzylorganotin compounds which contain both of the aforementioned favorable structures simultaneously are interesting as materials having a wide range of applications.
As specific chemical compounds which contain a carbon-carbon double bond, a phenyl group and an organotin bond are related to the vinyl-substituted benzylorganotin compounds, compounds having the following structures are known: ##STR2##
However, these compounds have many problems, such as reduced retention of the function after a long time and inferior compatibility with matrix polymer, because they are monomers. Therefore, they cannot be used satisfactorily in actual applications even though they have specific useful properties described above. Furthermore, single use of these tin compounds cannot satisfy requirements in diversifying fields.
In the field of polymers, rubber containing tin in the polymer chain has been attracting attention as materials satisfying various requirements.
In recent years, because lower fuel cost and safety are required for automobiles, lower rolling resistance (lower hysteresis loss), higher abrasion resistance and better wet skid resistance are desired for rubber compositions used in tread parts of tires.
For decreasing rolling resistance of rubber compositions used in tread parts of tires, properties of polymer contained in the rubber composition have been examined and natural rubber, polyisoprene rubber and polybutadiene rubber and the like have conventionally been used as the polymers having lower hysteresis loss. However, decreasing rolling resistance and increasing wet skid resistance contradict each other. When a rubber composition containing these polymers are used, wet skid resistance is decreased in the formulation by which rolling resistance can be decreased. In the formulation by which wet skid resistance is increased, rolling resistance of the rubber composition cannot be decreased because of an increase of a reinforcing agent, such as carbon black, and a plasticizer, such as oil, in the composition is required.
Styrene-butadiene copolymers containing different content of the bound styrene or different content of the vinyl linkage in the butadiene part were blended in a rubber composition to achieve a good property balance between abrasion resistance, rolling resistance and wet skid resistance of the rubber composition (Japanese Patent Publications 46-27069, 47-42729, 47-13532 and 47-17449 and Japanese Patent Application Laid-Open Nos. 55-60539, 56-163908, 57-70137 and 57-55941). However, an increase of the content of bound styrene could not decrease rolling resistance even though wet skid resistance was improved.
Furthermore, when the vinyl content of the butadiene part of styrene-butadiene copolymer in a rubber composition is increased, the fracture properties and the abrasion resistance are deteriorated even though the wet skid resistance of a tire using the rubber composition is improved. Therefore, it is clear from these results that a favorable rubber composition used for tire treads which satisfies the requirement to decrease rolling resistance (hysteresis loss) and other requirements in response to the requirement to decrease fuel cost of automobiles cannot be obtained merely by optimizing the content of bound styrene, the microstructure of the butadiene part, the distribution of composition in the molecular chain and the molecular weight distribution.
As a polymer having significantly decreased hysteresis loss for use in a rubber composition to satisfy these requirements, there has been disclosed a polymer which is prepared by the coupling of a halogenated tin compound with lithium at the end of the polymer prepared by polymerization using an organolithium compound as the initiator in a hydrocarbon solvent, utilizing the useful property of the tin compound described above (Japanese Patent Application Laid-Open No. 57-55912 and others). This polymer has very excellent physical properties and is used in a rubber composition for tires having low rolling resistance, accordingly, lower fuel cost.
However, in a conventional polymer obtained by coupling the lithium at the chain end of the polymer with a halogenated tin compound, the coupling reaction does not reach 100% as expected from the theoretical calculation even when the equivalent amounts of the halogenated tin compound and the lithium at the chain end are brought into reaction. In industrial processes, the reaction may only reach about 60%. Coupling efficiency tends to further decrease when the molecular weight of the polymer having lithium at the end is increased. Furthermore, tin is bonded only to the end of the polymer chain. It is impossible to introduce tin compounds into parts of the chain other than the chain end by this method. Accordingly, because low hysteresis loss depends on the amount of tin in the polymer, the heretofore known method cannot provide a polymer having sufficiently decreased hysteresis loss even though a polymer in which hysteresis loss has decreased to some degree can be provided.